In recent years, the demand for high tensile strength steel sheets (high tensile strength steel materials) which contribute to more lightweight structures and the like is increasing in the fields of automobiles, household appliances, building products, etc. As high tensile strength steel sheets, for example, it is known that a steel sheet with favorable hole expandability can be produced by containing Si in steel, and a steel sheet with favorable ductility where retained austenite (γ) forms easily can be produced by containing Si or Al in steel.
However, in the case of producing a galvannealed steel sheet using, as a base material, a high tensile strength steel sheet containing a large amount of Si (particularly, 0.2 mass % or more), the following problem arises. The galvannealed steel sheet is produced by, after heat-annealing the steel sheet as the base material at a temperature of about 600° C. to 900° C. in a reducing atmosphere or a non-oxidizing atmosphere, hot-dip galvanizing the steel sheet and further heat-alloying the galvanized coating.
Here, Si in the steel is an oxidizable element, and is selectively oxidized in a typically used reducing atmosphere or non-oxidizing atmosphere and concentrated in the surface of the steel sheet to form an oxide. This oxide decreases wettability with molten zinc in the galvanizing process, and causes non-coating. With an increase of the Si concentration in the steel, wettability decreases rapidly and non-coating occurs frequently. Even in the case where non-coating does not occur, there is still a problem of poor coating adhesion. Besides, if Si in the steel is selectively oxidized and concentrated in the surface of the steel sheet, a significant alloying delay arises in the alloying process after the hot-dip galvanizing, leading to considerably lower productivity.
In view of such problems, for example, JP 2010-202959 A (PTL 1) describes the following method. With use of a direct fired furnace (DFF), the surface of a steel sheet is oxidized and then the steel sheet is annealed in a reducing atmosphere to internally oxidize Si and prevent Si from being concentrated in the surface of the steel sheet, thus improving the wettability and adhesion of the hot-dip galvanized coating. PTL 1 describes that the reducing annealing after heating may be performed by a conventional method (dew point: −30° C. to −40° C.).
WO2007/043273 A1 (PTL 2) describes the following technique. In a continuous annealing and hot-dip coating method that uses an annealing furnace having an upstream heating zone, a downstream heating zone, a soaking zone, and a cooling zone arranged in this order and a hot-dip molten bath, annealing is performed under the following conditions to internally oxidize Si and prevent Si from being concentrated in the surface of the steel sheet: heating or soaking the steel sheet at a steel sheet temperature in the range of at least 300° C. by indirect heating; setting the atmosphere inside the furnace in each zone to an atmosphere of 1 vol % to 10 vol % hydrogen with the balance being nitrogen and incidental impurities; setting the steel sheet end-point temperature during heating in the upstream heating zone to 550° C. or more and 750° C. or less and the dew point in the upstream heating zone to less than −25° C.; setting the dew point in the subsequent downstream heating zone and soaking zone to −30° C. or more and 0° C. or less; and setting the dew point in the cooling zone to less than −25° C. PTL 2 also describes humidifying mixed gas of nitrogen and hydrogen and introducing it into the downstream heating zone and/or the soaking zone.
JP 2009-209397 A (PTL 3) describes the following technique. While measuring the dew point of furnace gas, the supply and discharge positions of furnace gas are changed depending on the measurement to control the dew point of the gas in the reducing furnace to be in the range of more than −30° C. and 0° C. or less, thus preventing Si from being concentrated in the surface of the steel sheet. PTL 3 describes that the heating furnace may be any of a direct fired furnace (DFF), a non-oxidizing furnace (NOF), and a radiant tube, but a radiant tube is preferable as it produces significantly advantageous effects.